As described in Patent Document 1, for example, electronic control units including first and second microcomputers (hereafter, referred to as micros) have been conventionally proposed. Each micro-computer incorporates a nonvolatile memory in which data can be rewritten and a communication circuit connected to a communication line. When each micro-computer receives data to be written addressed to itself through the communication circuit, it writes the data to be written to its nonvolatile memory to update the data stored in the nonvolatile memory.
In such an electronic control unit as mentioned above, the following may take place: when at least one of the multiple micro-computers updates the data stored in its nonvolatile memory, transition is caused from control mode in which all the micro-computers carry out control processing to rewrite mode in which data rewrite processing is carried out. That is, the electronic control unit may synchronize the multiple micro-computers with respect to operating mode. This is intended to suppress malfunction of the electronic control unit due to collapse of the monitoring system between the micro-computers.
Some electronic control units operate on different power supply systems. For example, some electronic control units operate on both the following power supply systems: a not-always-on power supply system in which a power source is turned on and off (in other words, power is supplied and power supply is stopped) by a user's manipulation; and an always-on power supply system in which power is constantly supplied. In such an electronic control unit, the following micro-computers may be provided: a micro-computer that operates on the not-always-on power supply system and a micro-computer that operates on the always-on power supply system.
When an electronic control unit is operating in rewrite mode, power supply to a micro-computer can be stopped. In this case, the following can take place in the electronic control unit that operates different power supply systems when power supply to the micro-computer is resumed and rewriting is carried out again: the timing of staring operation can differ between the micro-computer that operates on the not-always-on power supply system and the micro-computer that operates on the always-on power supply system. For this reason, the electronic control unit cannot synchronize the micro-computers with respect to operating mode and can malfunction.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2006-268107 (corresponding to US 2006/0227606 A1)